When the Appalachians were higher than the Rockies and North America was butting up against Africa, fearsome tectonic forces forged a kind of East Coast metamorphic rock known as schist.

Whole cities are built on schist.

In Philadelphia, you can drive through schist along the Schuylkill River. Kelly Drive bores straight through a big chunk of the stuff, presumably known as Glendinning Rock.

Two weeks ago I took a guided ramble on top of that rock with Scott Quitel, the curious and enthusiastic founder of LandHealth Institute.

We climbed over the Kelly Drive tunnel and stood even with Girard Avenue bridge, looking down at an overgrown park with a sign reading “Glendinning Rock Garden.”

“It’s amazing to think what was here before,” Scott said, pointing at stone foundations and old retaining walls. Vines sprouted from cement slabs. A spring cascaded down the hillside, bypassing the fountain designed to catch it.

The sign was ambiguous. Was the park originally designed as a rock garden? Or was it some other kind of garden, adjacent to Glendinning Rock?

But on the bare slope above, Scott pointed to ice plants scrabbling onto cracks in the sunlight. “These are the kind of plants you’d typically put in a rock garden. And no one planted them here—they're completely native.”